1. Field of the Invention
The present invention relates to a reaction apparatus used to effect reactions, e.g., reforming reactions of materials such as hydrocarbon, within reaction tubes.
2. Description of the Related Art
A reaction apparatus related to the present invention is schematically shown in section in FIG. 1. As shown in this figure, a reaction tube 200 of the reaction apparatus basically comprises a circular cylindrical inner tube 31, an outer tube 32 disposed concentrically with the inner tube 31, an intermediate tube 33 disposed between these inner and outer tubes 31 and 32 concentrically therewith, and an annular end-cap 38 disposed at the respective one ends of the inner and the outer tubes 31 and 32. A first annular portion 34 is defined between the inner tube 31 and the intermediate tube 33, and allows the introduction therein of a material such as a material gas 9. A catalyst 36 provided for use in reforming reactions is charged in the first annular portion 34 and thus forms a catalyst layer 35 therein. A second annular portion 37 is defined between the intermediate tube 33 and the outer tube 32, and allows the flowing therethrough of reformed gas 10 produced by effecting reforming reactions while allowing the material gas 9 to pass through the catalyst layer 35. The first annular portion 34 and the second annular portion 37 communicate with each other through the annular end-cap 38, in which the reformed gas 10 flowing from the catalyst layer 35 disposed in the first annular portion 34 is inverted and is thus caused to flow into the second annular potion 37. The reformed gas 10 then flows within the second annular portion 37 in the direction opposite to the direction of the flow of the material gas 9.
An annular pan 39 for holding the crystal 36 is provided at that end of the first annular portion 34 closer to the ends where the annular end-cap 38 is provided. The pan 39 is formed with a plurality of holes (none of which are shown) through which the gas flows. The annular end-cap 38 is surrounded by an annular end-cap insulator 40 provided on the outer side of the end-cap 38. The inside of the inner tube 31 defines a flow passage 42 through which a high-temperature combustion gas 41, serving as the heat source, flows. At least the side of the flow passage 42 that includes the exit of the combustion gas 41 is filled with filler particles 43 comprising, for instance, a ceramic or metallic material. The heat of the combustion gas 41 is transferred to the first annular portion 34 through a gas radiation portion 44 and a solid radiation portion 45.
FIG. 2 schematically shows the arrangement of a heating furnace in which a plurality of such reaction tubes 200 are disposed. In FIG. 2, the same reference numerals as those in FIG. 1 are used to designate the same or corresponding parts or components. Referring to FIG. 2, a plurality of reaction tubes 200 are disposed within a heating furnace 201. A burner 211 is disposed at one end of the heating furnace 201, while an inlet manifold 221 through which a material gas 9 is introduced into the heating furnace 201 is provided at the other end. Further, the heating furnace 201 is provided with an outlet manifold 231 for reformed gas 10 which communicates with each of the reaction tubes 200 and through which the reformed gas 10 is discharged, and an exhaust manifold 241 for a combustion gas 41 which communicates with each reaction tube 200 and through which the combustion gas 41 is exhausted after use. The wall of the heating furnace 201 is provided with a furnace wall insulator 251 which is shown in FIG. 2 as being integral with the wall of the furnace 201. In addition, heat insulators 46 are provided around the outer sides of the reaction tubes 200, more specifically, around the outer sides of the outer tubes 32. In the illustrated example, these heat insulators 46 are provided in the gaps between adjacent ones of the outer tubes 32 and in the gaps between the furnace wall insulator 251 and the adjacent outer tubes 32. With the above-described arrangement, therefore, the combustion gas 41 passes through the flow passages 42 alone, which are defined within the inner tubes 31. As a result, no combustion gas passes through the gaps between the outer tubes 32. In FIG. 2, the illustration of lines for supplying the burner 211 with gases required thereby, such as fuel and combustion air, is omitted.
Next, the operation of the above-described reaction apparatus will be explained. After hydrocarbon and steam, which form a material gas 9, have been preheated to a temperature of, e.g., about 450.degree. C., they are introduced into the heating furnace 201 through the inlet manifold 221. The material gas 9 is then introduced into the first annular portions 34 located between the inner tubes 31 and the intermediate tubes 33 of the reaction tubes 200, thereby allowing the material gas 9 to flow through the catalyst layers 35 formed within the first annular portions 34 and thus contact with the catalyst 36. In this process, the material gas 9 undergoes steam reforming reactions whereby the material gas 9 is transformed into reformed gas 10 which is a mixture of such gases as H.sub.2, CO, and CO.sub.2. After the completion of the reactions, the reformed gas 10, which is at a high temperature (e.g., about 800.degree. C.), passes through the gas-flow holes (not shown) of the annular pan 39, flows into the annular end-caps 38 in which the direction of flow of the reformed gas 10 is inverted, then flows into the second annular portions 37 located between the intermediate tubes 33 and the outer tubes 32. In this second annular portions 37, the reformed gas 10 flows in the direction opposite to that of flow of the material gas 9. In the process in which the reformed gas 10 flows through the second annular portions 37, heat transfer is proceeded between the reformed gas 10 and the intermediate tubes 33. After the sensible heat of the reformed gas 10 has been absorbed by the catalyst layers 35 through the walls of the intermediate tubes 33, the reformed gas 10 is discharged through the outlet manifold 231 to the outside of the system.
A combustion gas 41, serving as the heat source, is supplied by the burner 211 disposed within the heating furnace 201. The combustion gas 41 then flows through the flow passages 42 located inside the reaction tubes 200, more specifically within the inner tubes 31, along the inner walls of the inner tubes 31, whereby the wall portions of the inner tubes 31 that correspond to the gas radiation portions 44 are heated. After passing through the gas radiation portions 44, the combustion gas 41 flows through the solid radiation portions 45 filled with the filler particles 43, whereby the filler particles 43 are heated. Since the filler particles have a predetermined heat capacity, they emit solid radiation heat which is at a temperature level determined by the interrelation between their heat capacity and their emission capacity even when there are, e.g., reductions in the fuel flow rate. Consequently, the wall portions of the inner tubes 31 that correspond to the gas radiation portions 44 and the wall portions of the inner tubes 31 that correspond to the solid radiation portions 45 are substantially uniformly heated. The heat of the thus heated wall portions is used to heat the material gas 9 flows through the catalyst layers 35, as well as to heat the reformed gas 10. The heating of the material gas 9 determines the starting conditions of the catalytic reactions, whereas the heating of the reformed gas 10 determines the degree of progress of the catalytic reactions. By virtue of these factors, therefore, it is possible to stabilize the reaction conditions even with variations in the fuel flow rate. Incidentally, the end-cap insulators 40 are provided to prevent any heating of the portions which need not be heated by the combustion gas 41, more specifically, the inside of the annular end-caps 38 that is not charged with any catalyst 36.
As described above, since the combustion gas 41 passes solely through the inside of the inner tubes 31, various portions of the walls of the inner tubes 31 are uniformly heated by the gas radiation of the combustion gas 41 and the solid radiation of the filler particles 43, thereby enabling the catalyst layers 35 to be uniformly heated through the walls of the inner tubes 31 and, accordingly, enabling the reforming reactions to be effected uniformly. In addition, because the combustion gas 41 which has been supplied by the burner 211 flows through the inside of the inner tubes 31, it suffices if the space (the combustion space) within the heating furnace 201, which is filled with the combustion gas 41, has a dimension corresponding to the length of the burner flames.
Among various tube-wall portions of each reaction tube 200, the one that has the highest wall temperature is a wall portion of the inner tube 31 which is located most upstream of the flow of the combustion gas 41 and which is not covered with the end-cap insulator 40. Since this wall portion does not directly face the wall of the furnace 201, the solid radiation heat from the furnace wall can be neglected. This renders the gas radiation from the combustion gas 41 prevailing within the inner tube 31 predominant, and the combustion gas 41 thus makes it possible to effect uniform heating. On the other hand, the wall temperatures of the inner tubes 31, which may vary between the reaction tubes 200, can be made uniform by making uniform the amount of supply of the combustion gas 41 to each of the reaction tubes 200.
The above-described reaction apparatus, however, encounters various problems. For instance, since the filler particles 43 charged in the flow passages 42 provided for a combustion gas 41 and defined within the inner tubes 31 are heated by the combustion gas 41 serving as the heat source and being at a high temperature, the volume of the filler particles 43 is increased by thermal expansion. Thermal stress resulting from the increase in volume may act on the inner tubes 31, and may cause the inner walls of the inner tubes 31 to be deformed, or broken in the worst case. In some cases, there is a further risk that the filler particles 43 may become damaged.